Neither. Adele has RUINED her voice with heavy smoking and INcorrect, UNhealthy techniques which have forced her to cancel MANY of her gigs and Mariah is just moaning and groaning away (too much vocal acrobatics is just too much).

mariah used to have the "better" voice if you just want to talk about scales and high notes. she's never really done anything great though especially if you compare her to a powerhouse like whitney houston, who blows her out of the water in every respect. and she's totally lost her abilities now and is just sleep walking through her appearances.

adele on the other hand knows how to use her voice and has the gravitas of dramatic effect in her songs. she is a REAL singer and doesn't just practice going up and down the scales.